switch to sourcedir directive for loading servers from DHCP
    
    Drop the chrony-helper script in favor of the new sourcedir directive.
    
    Modify the dhclient script to save NTP servers from DHCP to
    /run/chrony-dhcp/$interface.sources and configure chronyd to (re)load
    the sources from files in that directory on start and the "reload
    sources" command.
    
    Other functionality of the helper script is dropped with no
    replacemement.